Re: [RFC PATCH 7/7] mm: zswap: Use acomp virtual address interface

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (25/03/04 16:42), Herbert Xu wrote:
> On Tue, Mar 04, 2025 at 05:33:05PM +0900, Sergey Senozhatsky wrote:
> >
> > And at some point you do memcpy() from SG list to a local buffer?
> > 
> > zsmalloc map() has a shortcut - for objects that fit one physical
> > page (that includes huge incompressible PAGE_SIZE-ed objects)
> > zsmalloc kmap the physical page in question and returns a pointer
> > to that mapping.
> 
> If the SG list only has a single entry, there will be no copies
> whatsoever even with the existing scomp code (crypto/scompress.c):

Nice.

[..]

> This still does an unnecessary copy for highmem, but that will
> disappear after my acomp patch-set:
> 
>                 if (sg_nents(req->src) == 1 &&
>                     (!PageHighMem(sg_page(req->src)) ||
>                      req->src->offset + slen <= PAGE_SIZE))
>                         src = kmap_local_page(sg_page(req->src)) + req->src->offset;
> 		else
> 			Use scratch buffer and do a copy
> 
> I've also modified LZO decompression to handle SG lists which I will
> post soon.  That will mean that no copies will ever occur for LZO
> decompression.  The same change could be extended to other algorithms
> if someone wishes to eliminate the copy for their favourite algorithm.

That's interesting.